In a height adjustment mechanism for a vehicle seat, a brake device is used which is configured such that an output shaft is caused to rotate by a swinging-up/swinging-down operation of a lever provided at an input side thereof but is prevented from rotating even when a force which is derived from the weights of the seat and an occupant seated thereon and by which the seat is biased downward is applied to the output shaft (Patent Literature 1).
In such a brake device, an outer race having a cylindrical inner peripheral surface, a plurality of brake cams facing this inner peripheral surface (clamping members in Patent Literature 1), and an output-side rotary member disposed inside the brake cams (a bolt having wings in Patent Literature 1) are provided. The rotational motion received by the brake cams is transmitted from the brake cams to cause a motion of the output-side rotary member, while if the output-side rotary member is caused to rotate, a force imparted to the brake cams by the output-side rotary member brought into contact with the brake cams acts mainly as a force pressing the brake cams against the outer race, and a rotational force imparted to the brake cams by the output-side rotary member cannot go beyond the frictional force which can be produced between the brake cams and the outer race, so that the brake cams cannot rotate.
In this brake device, two brake cams are arranged adjacently in a circumferential direction.